This invention relates to a radio frequency receiving system and, more particularly, to such a system wherein a phase modulated pseudonoise sequence signal is received.
In certain inter-vehicular communication systems, phase modulated pseudo-random noise coded pulses are used for both initial acquisition and data. An acquisition phase is used to acquire time synchronization and frequency correction of the received signal, after which data can be decoded by conventional means. The initial frequency uncertainty may be due to a combination of Doppler frequency shift due to vehicular motion and/or to local oscillator offset or drift. In such a system, it is typical that the vehicle wishing to receive data stepwise rotates its directional receiving antenna and examines any incoming signal to determine whether an appropriate signal is received. During the acquisition phase, the transmitter sends a phase modulated pseudonoise sequence signal, which is a specially coded signal. The receiver must therefore determine whether that coded signal has been received and, if so, whether the frequency of the received signal is such that it can be decoded without undue error.
It is therefore an object of the present invention to provide a low cost hardware implementation of a system for determining the acquisition of a phase modulated pseudonoise sequence signal when the time of arrival and frequency offset are unknown.
It is a further object of the present invention to provide such an arrangement which inherently compensates for frequency offset of the received signal so that errors due to frequency offset are reduced.
U.S. Pat. No. 5,285,472, the contents of which are hereby incorporated by reference, discloses a system which receives a pseudonoise sequence signal, quantizes the signal during a fixed interval of time and passes the quantized signal to a plurality of correlation channels. Each of the correlation channels rotates the phase of the quantized signal at a different fixed rate to cancel out a corresponding component of offset frequency. The outputs of the correlation channels are examined to determine whether acceptable correlation has been attained and, if so, which channel provides the best frequency compensation. The patented system enjoys a low cost, low complexity advantage by quantizing the signal prior to the determination and compensation of the Doppler shift. However, the use of multiple correlation channels rapidly becomes unworkable due to the growth in hardware complexity if longer codes (e.g., code length M&gt;32) or non-repeating codes are used.
It is therefore a more specific object of the present invention to provide a system of the type disclosed in the referenced patent which minimizes the growth in hardware complexity.